The gravitational lens B1938+666 as seen in the infrared when observed with the 10-meter Keck II telescope with Adaptive Optics on Mauna Kea, Hawaii. In the center is a massive red galaxy 9.8 billion light-years from Earth that acts like a cosmic magnifying glass, distorting the light from an even more distant galaxy, 17.3 billion light-years away. The result is a spectacular Einstein ring image of the background galaxy.
The team used distortions within the ring to find evidence for a low-mass dark galaxy, which is a satellite of the foreground lensing galaxy. Using this gravitational lensing effect the mass of the dark galaxy was found to be 200 million times the mass of the Sun, which is similar to the masses of the satellite galaxies found around our own Milky Way, but is 9.8 billion light-years further away.
credit: D. Lagattuta / W. M. Keck Observatory